1. Field of the Invention
The present invention relates generally to fiber-optic interferometric sensors and especially to apparatuses and methods for measuring the input phase shift between two interfering waves where the phase shift has been summed with a modulation phase shift, whose amplitude and phase may be precisely controlled such that subsequent demodulation provides a high accuracy measure of the input phase, being especially useful when multiple element sensor arrays are interrogated using Time Division Multiplexing (TDM)techniques.
2. Description of Prior Art
Fiber optic interferometers are implemented in sensors and sensor arrays to measure physical properties such as acoustic level, pressure, acceleration, temperature, electric and magnetic fields. Numerous techniques described in the literature have been applied to the design of interferometric fiber sensor array systems. A principal objective of such designs is to obtain high quality measurements of the desired physical parameter. A second objective is to evolve designs that are cost effective without compromising the quality of the measurement data.
Regarding TDM sensor array approaches, general design practices involve a passive remote sensor array comprised of path mismatched Single Mode Fiber (SMF) interferometers and an xe2x80x9cinterrogator.xe2x80x9d The interrogator contains the active components. A typical interrogator design contains elements that optically feed the sensor array, and elements which process the optical signals returning from the array.
Interrogator elements which feed the array include: (1) a CW laser; (2) an optical pulser; and (3) an optical phase or frequency modulator which is sometimes housed in a path mismatch compensating interferometer, where timing and control electronics are used to sequence the pulses and control the modulation signal. Interrogator elements which process the array returns include: (1) a polarization diversity receiver to insure high interferometric visibility; (2)signal conditioning and sampling electronics; (3) demodulation electronics; and (4)timing electronics to coordinate the sampling process of the TDM returns.
The interrogator feed elements, due to functional necessity, are highly specialized. In general, TDM interrogation mandates that high-extinction, precisely modulated optical pulses (typically 2 xcexcsec or less in duration) be created and submitted to the sensor array. In order to produce the phase modulation, high frequency Integrated Optic Chip (IOC) phase modulators which implement Polarization Maintaining (PM) fiber inputs become the device of choice. Since the modulation is created subsequent to the optical pulse creation, the necessity of polarized operation is also extended to the pulser, which typically is also an IOC device. Here a highly specialized dual Mach Zehnder element is used to meet the high optical extinction requirements, which are typically 100,000:1 or higher. Additionally specialized servo electronics are required for the integrated optic pulser to insure it is maintaining a high optical extinction. If the interrogator design implements a compensating interferometer, which most do, then this element also requires PM input splitting components.
The use of PM fiber, customized integrated optic components and specialized electronic circuitry tend to drive the manufacturing costs of interrogators to high levels. These high costs present a disadvantage to general industry acceptance.
It is an aim of the present invention to reduce the inherent design cost of interrogators without compromising measurement performance.
The present invention is an interrogator with reduced design complexity and specialization. Features of the present invention include a topology that need only use single mode fiber (SMF) components and in the preferred embodiment of the modulation, use a fiber stretcher and a Faraday Rotating Mirror (FRM) within a compensating interferometer. Additionally, the preferred embodiment uses self-correcting processing approaches that reduce modulation errors to provide high accuracy measurements and significantly reduce the cost of interrogation. A general objective of the present invention is to reduce the complexity and cost of interrogators for multi-channel fiber sensor systems.
The present invention comprises an apparatus and method for low-cost TDM interrogations within high performance fiber interferometric sensor array systems. The invention uses a new xe2x80x9cdual slopexe2x80x9d modulation/demodulation approach which is capable of high performance TDM interrogation without the requirement of high frequency or polarization-maintaining modulators. This resulting topology in turn enables the optical design of the interrogator to be completely free of costly polarization-maintaining elements.
For example, the preferred embodiment of the interrogator feed elements consist of a CW laser diode which is input to a SMF fiber coupled Acousto-Optic pulser, which is then input to a SMF compensation interferometer containing a delay fiber with reflective termination in one leg and a SMF wrapped piezoelectric fiber stretcher (modulator) with FRM termination in the other leg. Such an assembly for the interrogation feed can be manufactured at significantly lower relative cost than prior art configurations using PM elements with integrated optic modulators and pulsers.
The key object to this invention is to create alternating and opposite modulation phase slopes such that each has a linear excursion of 2 pi radians within the duration of an optical pulse used to interrogate the sensor array. The return optical pulse from each sensor in the array is sampled five times for each slope such that each sample represents a phase slope excursion of xcfx80/2. The five-samples-per-pulse process embodiment, by way of example and not limitation is summarized as follows:
(a) S0, S1, S2, S3 and S4 are samples taken within a pulse length from the modulated up slope;
(b) S5, S6, S7, S8 and S9 are samples taken within a successive pulse length from the modulated down slope;
(c) Quadrature terms generated from the samples are from the upslope and corresponding phase are:
cos(RPSS)=2S2xe2x88x92S4xe2x88x92S0;
sin(RPSS)=2(S1xe2x88x92S3); and result in a fringe-corrected output phase for the positive slope samples, RPSS:
RPSS=tanxe2x88x921(sin(RPSS)/cos(RPSS)+fringe);
(d) Quadrature terms generated from the samples are from the downslope and corresponding phase measured are
cos(RNSS)=2S7xe2x88x92S5xe2x88x92S9;
sin(RNSS)=2(S8xe2x88x92S6); and result in a fringe-corrected output phase for the negative slope samples, RNSS:
RNSS=tanxe2x88x921(sin(RNSS)/cos(RNSS)+fringe);
(e) The truer measure of the output phase signal, R(t), during the modulation cycle cancels any modulation artifacts and is the sum of the above two derived measurements
R(t)=xc2xd(RPSSxe2x95x90RNSS);
(f) The modulation phase error, We, is used as a modulation feedback term to correct the time sequencing of the slopes and is given by
We=xc2xd(RPSSxe2x88x92RNSS);
(g) The modulation amplitude error, EM, is used as a modulation amplitude feedback correction term to insure the sampling sequence S0xe2x88x92S4 and S5xe2x88x92S9 occur at precise intervals of xcfx80/2. The measure of this error is quadrant sensitive and is given for all four positive slope cases, EMP, by
EMP,0=S1xe2x88x92S2+S3xe2x88x92S4, quadrant 0;
EMP,1=S0xe2x88x92S1+S2xe2x88x92S3, quadrant 1;
EMP,2=xe2x88x92EMP,0=xe2x88x92S1+S2xe2x88x92S3+S4, quadrant 2; and
xe2x80x83EMP,3=xe2x88x92EMP,1=xe2x88x92S0+S1xe2x88x92S2+S3, quadrant 3;
and all four negative slope cases, EMN, by:
EMN,0=xe2x88x92S5+S6xe2x88x92S7+S8, quadrant 0;
EMN,1=xe2x88x92S6+S7xe2x88x92S8+S9, quadrant 1;
EMN,2=xe2x88x92EMN,0=+S5xe2x88x92S6xe2x88x92S7xe2x88x92S8, quadrant 2; and
EMN,3=xe2x88x92EMN,1=+S6xe2x88x92S7+S8xe2x88x92S9, quadrant 3.
An appreciation of the present invention and a more complete understanding of its structure and method of operation may be had by studying the following description of the detailed description with preferred embodiment and by referring to the accompanying drawings.